Welcome to the Thunder Dome

Welcome to the Thunder Dome

Atlanta's Urban Heat Alters Weather
Patterns

April 26, 1999: As the heat builds during a blistering
summer day in Atlanta, Georgia, you can almost hear the clouds
overhead cry, "Let's get ready to rumble!"

Urban growth has transformed Atlanta's environment, creating
a uniquely altered arena of weather. Because urban areas both generate
and trap heat, a bubble or "urban heat island" forms
around the city. The temperature in Atlanta is 5 to 8 degrees
Fahrenheit higher than outlying areas, and this excess heat produces
increased rainfall and thunderstorms.

This finding was presented at the annual meeting
of the Association of American Geographers in Honolulu, Hawaii
on March 24 by meteorologists Robert Bornstein and Qing Lu Lin
from San Jose State University in California. Dale Quattrochi
and Jeffrey Luvall of NASA's Global Hydrology Center lead this
NASA-sponsored study . The Atlanta Land-use Analysis: Temperature
and Air-quality (ATLANTA) project began in 1996 in order to study
the impact of urban heat islands on the environment.

As the heat in a city builds, hot air rises. Colder air rushes
into the vacuum, creating winds. The warmer ascending air forms
clouds that
drop water as they continue to rise. Bornstein and Lin found
that Atlanta's urban heat island causes convective clouds to
form over the city.

Left: Turbulent gust front clouds. Photo credit: NOAA
Photo Library.

"Convective clouds typically produce rains that are intense
and localized," says Bornstein. "These types of clouds
should also produce thunder and lightning."

Bornstein and Lin used data collected by the National Weather
Service and the Georgia Automated Environmental Network. They
also used data from the Geostationary-orbiting Operational Environmental
Satellite (GOES), which monitored Atlanta's weather conditions
during the 1996 Summer Olympics.

Southern cities in the United States usually experience summer afternoon
thunderstorms. Atlanta's storms, however, are out of the ordinary.
The Project ATLANTA team found that storms around Atlanta were
generated during heat island periods. Rather than only experiencing
afternoon storms, Atlanta would also experience pre-dawn or early
morning rain showers that would continue until noon.

"There's no doubt when you look at the patterns of precipitation
development that the showers were forced, or created by the 'sucking
in' of cooler air into the urban heat island over Atlanta,"
says Quattrochi.

Traffic versus Trees

Heat islands are created through the process of urbanization.
As a city grows, trees are cut down to make room for commercial
development, roads, and suburban growth. Forest growth normally
reduces the amount of heat and smog generated by populated areas.
Plants and water-retaining soils absorb heat during the day,
and then carry the heat away through evaporation. In Atlanta,
commercial and suburban development dramatically increased between
1973 and 1992, and nearly 380,000 acres of forest were cleared
to accommodate that growth.

Right: Heat builds in a city when forests are cut down
to build roads. Not only do paved roads hold in heat, but cars
compound the problem by generating smog and more heat (Below).
Photo credits: Department of Energy, Warren Gretz.

The materials used to build over these forests compound the
urban overheating problem. Asphalt roads, tar roofs, and other
dark, heat-absorbing materials hold in heat long after the sun
sets, keeping the cities hotter for longer periods of time. Atlanta
experiences early morning rain showers because urban heat islands
retain their temperature long after nightfall.
This rise in temperature also increases the amount of air pollution.
Not only is heat and pollution produced from automobiles and
commercial facilities, but Atlanta's 5 to 8-degree rise in temperature
contributes to an increase in ozone, a particularly destructive
type of smog. Ozone interferes with photosynthesis, the process
by which plants make food, and it damages the lungs of humans
and animals, sometimes causing permanent lung damage. As heat
levels rise, the city environment becomes increasingly hazardous.

Thunderstorms may be nature's way of keeping its cool. The
storms also help clean the air because the fresh rainfall acts
like a scrub-brush on air pollution. On the downside, thunderstorms
can cause flooding in urban areas because paved ground doesn't
allow water to soak into the soil.

Project ATLANTA often uses images from ATLAS, the Airborne
Thermal and Land Applications Sensor, which is onboard a Lear
23 jet. These images help scientists clearly see urban growth
trends. For instance, ATLAS images indicate that interstate and
other major highways generate urban sprawl. The images also show
temperature variations, color coding hot and cold areas for easy
recognition.

"This is the same basic instrument that Galileo has,"
Luvall says, referring to the unmanned NASA spacecraft that orbits
Jupiter.

ATLAS 'sees' in 15 colors, as well as in three visible light,
six near and middle-infrared, and six thermal infrared channels.
The ATLAS data collected over Atlanta has a resolution of 10
meters (33 feet), which means objects the size of an average
house are easily recognizable.

ATLAS' thermal data show that temperatures in parking lots
can reach 120 degrees Fahrenheit during the day, while the tree islands
in the same parking lot reach only 89 degrees. And the parking
lot retains that heat much longer than do the tree islands or
wooded areas.

Right: An ATLAS image of Atlanta, Georgia.

The study has helped bring attention to the problems of urban
sprawl in Atlanta. Because Georgia Governor Roy Barnes has shown
interest in the work, the scientists hope he will use the data
to initiate legislative action to support the use of tree planting
and installation of highly reflective rooftops to help cool Atlanta
and other major Georgia cities.

"Planting trees to shade the city and installing highly
reflective roofing materials are seen as measures that are politically
palatable as opposed to passing more restrictive air quality
legislation," says Quattrochi.

Already,
the Georgia government has eased insulation requirements for
buildings constructed with highly reflective roofs. The scientist's
study has also been embraced by high-reflecting roofing material
manufacturers, who use the data to illustrate how such materials
can keep a building cool while keeping energy costs down.

Although Quattrochi and Luvall have collected data for EPA
studies in many other cities, Project ATLANTA is unique. By looking
at changes in urban growth over the past 25 years, the scientists
have been able to monitor changes both in the area's environment
and meteorology. Quattrochi says he hopes to duplicate Project
ATLANTA in Houston beginning next summer. Because Houston's location,
vegetation and weather conditions are different from Atlanta's,
the scientists are interested to see whether Houston experiences
similar urban heat island effects.

"There are a lot of complicating factors to consider,"
says Quattrochi. "But my suspicion is if the city is large
enough, it probably has some impact on local weather."